目录
一，基本流程
二，消息发送
三，动态方法解析
四，消息转发
一，基本流程
1，方法的调用都会转换为objc_msgSend函数的调用，通常称为消息机制
// OC代码
[person eat];
// 底层代码（用clang进行转换）
((void (*)(id, SEL))(void *)objc_msgSend)((id)person, sel_registerName("eat"));
// 简化代码
objc_msgSend(person, @selector(eat)); // person为消息接收者，eat为消息名称
2，objc_msgSend执行流程
执行流程
二，消息发送
1，底层代码（源码下载地址）
IMP lookUpImpOrForward(Class cls, SEL sel, id inst,
bool initialize, bool cache, bool resolver)
{
IMP imp = nil;
bool triedResolver = NO;
retry:
runtimeLock.assertLocked();
// Try this class's cache.
// 查找自身的缓存
imp = cache_getImp(cls, sel);
if (imp) goto done;
// Try this class's method lists.
// 查找自身的方法列表
{
Method meth = getMethodNoSuper_nolock(cls, sel);
if (meth) {
// 找到就放入自身的缓存中
log_and_fill_cache(cls, meth->imp, sel, inst, cls);
imp = meth->imp;
goto done;
}
}
// Try superclass caches and method lists.
// 查找父类的缓存和方法列表
{
unsigned attempts = unreasonableClassCount();
// 遍历父类
for (Class curClass = cls->superclass;
curClass != nil;
curClass = curClass->superclass)
{
// Halt if there is a cycle in the superclass chain.
if (--attempts == 0) {
_objc_fatal("Memory corruption in class list.");
}
// Superclass cache.
// 查找父类的缓存
imp = cache_getImp(curClass, sel);
if (imp) {
if (imp != (IMP)_objc_msgForward_impcache) {
// Found the method in a superclass. Cache it in this class.
// 找到就放入自身的缓存中
log_and_fill_cache(cls, imp, sel, inst, curClass);
goto done;
}
else {
// Found a forward:: entry in a superclass.
// Stop searching, but don't cache yet; call method
// resolver for this class first.
break;
}
}
// Superclass method list.
// 查找父类的方法列表
Method meth = getMethodNoSuper_nolock(curClass, sel);
if (meth) {
// 找到就放入自身的缓存中
log_and_fill_cache(cls, meth->imp, sel, inst, curClass);
imp = meth->imp;
goto done;
}
}
}
// 这里省略了动态方法解析和消息转发的代码
done:
runtimeLock.unlock();
return imp;
}
2，图解
消息发送
3，说明
如果调用实例方法，就在
class对象的缓存和方法列表中查找；如果调用类方法，就在meta-class对象中查找方法列表指的是
class_rw_t里的methods如果方法列表已排序，就用二分查找法；如果没有排序，就用顺序查找法
三，动态方法解析
1，实例代码
// Person
@interface Person : NSObject
- (void)eat;
@end
@implementation Person
- (void)run {
NSLog(@"Person run");
}
// 添加实例方法需实现此方法，添加类方法需实现resolveClassMethod
+ (BOOL)resolveInstanceMethod:(SEL)sel {
if (sel == @selector(eat)) {
Method runMethod = class_getInstanceMethod(self, @selector(run));
// 动态添加eat的实现
class_addMethod(self,
@selector(eat),
method_getImplementation(runMethod),
method_getTypeEncoding(runMethod));
return YES;
}
return [super resolveInstanceMethod:sel];
}
@end
// 使用
Person *person = [Person new];
[person eat];
// 打印
Person run
2，底层代码
IMP lookUpImpOrForward(Class cls, SEL sel, id inst,
bool initialize, bool cache, bool resolver)
{
IMP imp = nil;
bool triedResolver = NO;
// 这里省略了消息发送的代码
// No implementation found. Try method resolver once.
// triedResolver为NO，未动态方法解析
if (resolver && !triedResolver) {
runtimeLock.unlock();
// 调用resolveInstanceMethod或resolveClassMethod
resolveMethod(cls, sel, inst);
runtimeLock.lock();
// Don't cache the result; we don't hold the lock so it may have
// changed already. Re-do the search from scratch instead.
// 标记已动态方法解析
triedResolver = YES;
// 重新执行消息发送
goto retry;
}
// 这里省略了消息转发的代码
done:
runtimeLock.unlock();
return imp;
}
3，图解
动态方法解析
4，说明
手动实现
resolveInstanceMethod或resolveClassMethod，并且在其中动态添加方法，是动态方法解析的关键动态添加的方法会存入
class_rw_t里的methods中，所以需要重新执行消息发送在消息发送过程中能否找到方法取决于是否有动态添加方法
四，消息转发
1，不开源
IMP lookUpImpOrForward(Class cls, SEL sel, id inst,
bool initialize, bool cache, bool resolver)
{
IMP imp = nil;
bool triedResolver = NO;
// 这里省略了消息发送和动态方法解析的代码
// No implementation found, and method resolver didn't help.
// Use forwarding.
// 无法查看内部具体实现
imp = (IMP)_objc_msgForward_impcache;
cache_fill(cls, sel, imp, inst);
done:
runtimeLock.unlock();
return imp;
}
2，第一个阶段
- 实例代码
// Person
@interface Person : NSObject
- (void)eat;
@end
@implementation Person
- (id)forwardingTargetForSelector:(SEL)aSelector {
if (aSelector == @selector(eat)) {
// 将eat转发给Dog去处理
Dog *dog = [Dog new];
return dog;
}
return [super forwardingTargetForSelector:aSelector];
}
@end
// Dog
@interface Dog : NSObject
- (void)eat;
@end
@implementation Dog
- (void)eat {
NSLog(@"Dog eat");
}
@end
// 使用
Person *person = [Person new];
[person eat];
// 打印
Dog eat
- 伪底层代码
if (class_respondsToSelector(receiverClass, @selector(forwardingTargetForSelector:))) {
id forwardingTarget = [receiver forwardingTargetForSelector:sel];
if (forwardingTarget && forwardingTarget != receiver) {
// forwardingTarget是返回的dog，sel是eat
return objc_msgSend(forwardingTarget, sel, ...);
}
}
3，第二阶段
- 实例代码
@implementation Person
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
if (aSelector == @selector(eat)) {
// 返回eat的方法签名，告知系统需要转发eat，系统就会调用forwardInvocation
return [[Dog new] methodSignatureForSelector:@selector(eat)];
}
return [super methodSignatureForSelector:aSelector];
}
- (void)forwardInvocation:(NSInvocation *)anInvocation {
// 将eat转发给Dog去处理
Dog *dog = [Dog new];
[anInvocation invokeWithTarget:dog];
}
@end
// 打印
Dog eat
- 伪底层代码
if (class_respondsToSelector(receiverClass, @selector(methodSignatureForSelector:))) {
// 调用methodSignatureForSelector
NSMethodSignature *methodSignature = [receiver methodSignatureForSelector:sel];
if (methodSignature && class_respondsToSelector(receiverClass, @selector(forwardInvocation:))) {
NSInvocation *invocation = [NSInvocation _invocationWithMethodSignature:methodSignature frame:frameStackPointer];
// 调用forwardInvocation
[receiver forwardInvocation:invocation];
void *returnValue = NULL;
[invocation getReturnValue:&value];
return returnValue;
}
}
4，图解
消息转发
5，说明
forwardInvocation只要实现了就不会抛出异常，即使在该方法中什么也不做如果转发的是类方法，需要将上面三个方法改成类方法，另外
target也要设置为类对象
// 类方法
+ (id)forwardingTargetForSelector:(SEL)aSelector {
if (aSelector == @selector(eat)) {
// 类对象
return [Dog class];
}
return [super forwardingTargetForSelector:aSelector];
}
-
NSInvocation是对将被转发消息的封装
// Person
@interface Person : NSObject
- (int)eat:(int)food;
@end
@implementation Person
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
if (aSelector == @selector(eat:)) {
return [[Dog new] methodSignatureForSelector:@selector(eat:)];
}
return [super methodSignatureForSelector:aSelector];
}
- (void)forwardInvocation:(NSInvocation *)anInvocation {
NSLog(@"消息接收者---%@", anInvocation.target);
NSLog(@"消息---%@", NSStringFromSelector(anInvocation.selector));
int argument;
[anInvocation getArgument:&argument atIndex:2]; // 系统默认会添加两个参数
NSLog(@"参数---%d", argument);
int returnValue;
[anInvocation getReturnValue:&returnValue];
NSLog(@"返回值---%d", returnValue);
}
@end
// 使用
Person *person = [Person new];
[person eat:1];
// 打印
消息接收者---<Person: 0x100613070>
消息---eat:
参数---1
返回值---0
